Photosensitive etch resistant films based on aqueous solutions of natural products such as fish glue, albumin, and casein have been known for a number of years. One such useful photoresist composition comprises water, casein, made by acid precipitation of milk, an alkali metal base to impart a pH of 7.7 or higher which improves the solubility of the natural product in the water, and an ammonium dichromate sensitizer. Goldman and Datta in their U.S. Pat. No. 4,061,529 disclose the addition of sodium borate as the base in a concentration such that the photoresist solution has the final pH of 6.7 to 7.3, to improve the pot life of the coating composition and the shelf life of coated substrates.
These resists have been employed extensively in the lithographic printing arts and in the manufacture of shadow masks for television monitors. A cleaned metal, e.g., copper, surface or substrate is coated with the aqueous photoresist solution and dried. A mask having the desired pattern is contacted to the resist and exposed to light of an appropriate wavelength which hardens the resist in the exposed areas. The resist film is then washed with water to dissolve the unexposed resist and thereby uncover part of the metal surface. The now partially coated substrate is then dried and baked for about 5 minutes at a temperature of from about 260.degree.-287.degree. C. This step is required to render the remaining photoresist etch resistant. The partially coated substrate is then etched by spraying with hot ferric chloride solution, which etches away the bared metal portions. The residual resist can then be removed by hot alkali solution.
The above process has certain limitations which restrict its use in other high production manufacture which employ lithographic techniques; in particular, in the manufacture of printed circuit boards wherein a copper-clad substrate, such as a phenolic impregnated paperboard, is etched to form a pattern of conductors to which various components are soldered. Generally, these printed circuit boards are made by screen printing an etch resistant ink onto the boards and etching the exposed metal areas. However, this method is not able to define line widths and spacings sufficiently small as is now demanded by the increasing miniaturization of components on printed circuit boards. Photolithographic techniques to define such fine pattern spacings must then be employed if high yields are to be obtained.
The pattern is made by applying a photoresist film over the copper layer, exposing and developing the resist to create a pattern of photoresist and exposed copper. The copper is etched away in the exposed areas, and the photoresist is removed, leaving a patterned copper layer on the board. At present, organic-based photoresists are used because the high temperatures required to cure water-based resists cannot be tolerated by the phenolic impregnated paper substrate.
One attempted method to deal with this high temperature curing problem of the water-based photoresist is disclosed by Goldman in U.S. Application Ser. No. 877,481, filed Feb. 13, 1978, now U.S. Pat. No. 4,158,566, wherein it is disclosed to use an accelerator with such water-based photoresist compositions. As an accelerator there is disclosed N-methylol acrylamide which, when added to a casein-based photoresist composition, lowers the curing temperature required to make the photoresist etch resistant to a temperature of about 125.degree.-135.degree. C., and enables such compositions to be employed in printed circuit board manufacture.
Another method dealing with the etch resistance of water-based photoresists is disclosed by Dougherty in U.S. Application Ser. No. 967,793, filed Dec. 8, 1978, herein incorporated by reference, wherein it is disclosed that such photoresists are etch resistant or sufficiently etch resistant to allow the use of the same in the manufacture of printed circuit boards if certain process parameters are maintained. These parameters include the use of a minimum thickness (4.0 micrometers or above) of photoresist film, the use of a specific type (low hardness content) of water in developing the patterned photoresist, and the use of an etchant solution having a certain minimum specific gravity (1.34 or above).
There is a continuing need for improved etch resistance in water-based photoresists such that etchant solutions of lower specific gravity may be used. Lower specific gravity etchant solutions make possible increases in etch rates. This, then, increases the overall throughput in the manufacturing process, be it printed circuit boards or television shadow masks.